Gas diode having auxiliary cathode for fault currents



H. C. STEINER July 6, 1965 GAS DIODE HAVING AUXILIARY CATHODE FOR FAULT CURRENTS Filed Jan. '7, 1963 FIG.|.

INVENTORI HARRY c. STEINER, BW

HIS ATTORNEY.

United States Patent 3,193,718 GAS DIODE HAVING AUXHLIARY CATHGDE FUR FAULT CNTS Harry C. Steiner, Seotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 7, 1963, Ser. No. 249,649 Claims. (Cl. 313-189) My invention relates to electric discharge devices and pertains more particularly to a new and improved gaseous power rectifier tube including new and improved means for increasing the fault current capacity of the tube.

Gaseous rectifier tubes are generally designed for given fault current ratings; and the cathodes as well as the other electrodes of such devices are generally adapted for withstanding arc backs or other fault current conditions Within such ratings. However, under certain conditions of fault operation the current can exceed the fault current ratings by substantial amounts. This can cause sputtering, burning and other destructive effects on the cathode with the undesirable results of reduced reliability and shortened tube life. Generally these destructive effects can be overcome by redesigning the cathode to withstand the higher fault currents. However, this manner of coping with the problem is not desirable inasmuch as it would affect the operating characteristics of the tube and would necessitate other substantial design changes.

Accordingly, the primary object of my invention is to provide a new and improved gaseous rectifier tube including new and improved means for increasing the fault current capacity thereof.

Another object of my invention is to provide new and improved means for increasing the fault current capacity of a gaseous rectifier tube Without the necessity of modifying the cathode per se or effecting any other design changes in the tube and without affecting the operating charact-eristics of the tube.

Another object of my invention is to provide a new and improved gaseous rectifier tube adapted for increased reliability and extended operating life.

Another object of my invention is to provide new and improved means for increasing the fault current capacity of a gaseous rectifier tube which is relatively simple in construction and does not add appreciably to the cost or efforts of manufacture.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming part or" this specification.

In carrying out the objects of my invention I provide a gaseous rectifier tube comprising an envelope containing a mercury vapor atmosphere, an anode and a cooperating cathode structure. The cathode structure comprises a filamentary thermionic cathode having an alkaline earth metal oxide coating and mounted between a pair of support rods in an open-ended heat shield. Also mounted in the shield on the support rods are a pair of refractory metal cold cathodes each including a portion extending in the arc stream intermediate the thermionic cathode and the open end of the shield. During operation the cold cathode elements become coated with an oxide vaporized from the thermionic cathode. During fault current conduction a substantially dense ion plasma occurs in the region of the cold cathode elements and the oxide coating on such elements adapts them for collecting ions from the plasma. Thusly, the cold cathode elements are adapted for the establishment thereon of arc, or cathode, spots effective for attracting a portion of the arc during are back conditions and thereby relieving the thermionic cathode of some of the duty of supporting the entire anode current.

3,1Q3,7l8 Fatented July 6, 1965 For a better understanding of my invention reference may be had to the accompanying drawing wherein:

FIGURE 1 is a partially sectionalized view of a gaseous power rectifier tube incorporating an embodiment of my invention;

FIGURE 2 is an enlarged sectionalized view of the cathode structure employed in the device of FIGURE 1; and

FIGURE 3 is an enlarged fragmentary sectionalized side view of the cathode structure.

Referring to FIGURE 1, there is illustrated a gaseous electric discharge device comprising an elongated glass envelope 1 which is of a hermetically sealed construction. The envelope ll contains a predetermined quantity of an ionizable medium which can advantageously be an inert gas, a vapor-producing substance such as mercury or a combination thereof. When mercury is used, it can be the gas-filled space there is provided an anode 3 which is represented as being in the form of a bell-shaped graphite element. The anode 3 is mounted on a conductive lead supported from an anode contact 4 sealed to one end of the envelope. Internally of the envelope 1 the anode 3 is cooperatively positioned over the upper end of a cathode structure generally designated 5.

As perhaps better seen in FIGURES 2 and 3, the cathode structure 5 includes a metal heat shield 6 housing a filamentary thermionic cathode 7. The heat shield 6 comprises a metal cylinder 8 having upper and lower headers Zia and 85, respectively. The upper header 3a includes an enlarged central aperture 9 for allowing an arc stream to extend therethrough and between the anode 3 and the thennionic cathode 7. The lower header 8b is appropriately apertured for the extension therethrough of a pair of spaced cathode support rods 11 which extend parallel to each other and to the longitudinal axis of the tube. The support rods 11 serve for effecting electrical connecti'ons to the thermionic cathode, supporting the thermionic cathode in the shield 6 and assisting in supporting the complete cathode structure 5 in the cooperative position with respect to the anode 3; The rods 11 are suitably conductively connected to rigid internal leads 12 which extend in a hermetically sealed manner through a stem press 13 sealed in the lower end of the envelope 1. Also serving to support the cathode structure 5 is a support rod 14 which has the upper end thereof suitably secured to the lower shield header 8b and the lower end thereof securely imbedded in the stem press 15. Mounted on the internal leads 12 between the cathode structure and stem press is a disc-shaped heat shield 15 which supports a conventional getter structure 16. Externally of the tube envelope are provided a tube base 17 and flexible leads 18 connected to external portions of the leads 12.

The thermionic cathode 6 comprises an elongated sheet 19 of a refractory-met-al-mesh wound spirally about an insulative sleeve 20 fitted on a rod 21 suitably supported horizontally between the support rods 11. The mesh 20 is impregnated with a sintered alkaline earth metal oxide which preferably is barium-strontium-calcium oxide and which serves as the electron source. The ends of the mesh are each conductively connected to one of the support rods 11 which are insulated from the shield 6 by insulative seals 22 fitted in the apertures in the lower header 8b through which the support rods 11 extend. Thusly, an energizing circuit can be completed through the rods 11 and the cathode filament. The insulative sleeves 22 insulate the rods 11 from the shield 6. However, the shield is maintained at cathode potential by a center tap 23 conductively interconnecting the mid-section of the thermionic cathode and the lower header 3b of the shield.

During normal operation of the tube an arc stream extends through the aperture 9 in the upper header of the heat shield and between the anode 3 and the thermionic cathode '7. The thermionic cathode '7 is designed to withstand the current in the stream during such normal operation. Additionally, it is designed to withstand, without substantial sputtering or other damage, the effects of arc backs caused byfault current conduction within a given design fault current range. The present invention adapts the device for operating under fault current conditions which are outside the design fault current range and which ordinarily would effect sputtering and burning of the filamentary cathode. Specifically, the present invention involves the provision of a secondary, or cold, cathode structure in cooperative association with the thermionic cathode and adapted for serving as an electron source for fault currents and thereby effective for relieving the thermionic cathode of some of the duty of supplying the entire anode current during fault current operation.

According to the present invention, the cold cathode structure comprises a pair of inverted L-shaped electrode elements 25 which shall hereinafter be referred to simply as arcing horns. The arcing horns are formed of a refractory metal such, for example, as tungsten, molybdenum, tantalum or nickel and are each conductively secured as by means of a metal sleeve 26 to the upper end of one of the cathode support rods 11. The arcing horns include end portions 27 which extend horizontally and parallel to the axis of the spiral thermionic cathode. Additionally, the end portions 27 protrude into the arc stream extending between the thermionic cathode and the anode during conduction.

During normal current conduction the arcing horns as- 1 sume an elevated temperature approaching a dull red.

Also, during normal operation oxide evaporated from the thermionic cathode condenses on the arcing horns and thereby forms thereon insulative coatings which are particularly adapted for collecting ions. Accumulation of these ions on discretesections of the insulative coatings provide voltage gradients for effecting establishment of cathode spots, or emissive regions, on the arcing horns. Thusly, the horns are adapted for serving as an efficient emitter. More specifically, during fault current conduction the plasma ion density becomes high in the heat shield 6 and particularly in the region immediately adjacent the thermionic cathode and surrounding the portions 27 of the arcing horns. The increase in ion density prevalent during fault conditions serves to promote the rapid formation of the mentioned cathode spots on the arcing horns. Thus cathode spots form soon after a fault current appears and are subsequently effective for establishing arcs to the arcing horns with the desirable result that the fault currents tend to be supported from the cathode spots on the horns for relieving the thermionic cathode of some of the duty of supplying the entire anode current. In this manner, the thermionic cathode is protected from the damaging effects possible were it required to bear the full fault current.

It is to be understood from the foregoing that the arcing horn portions 27 are predeterminedly located in the shield 6 to dispose them in the mentioned high ion density region. It is also to be understood from the foregoing that while I have shown the filament-protecting structure as comprising a pair of arcing horns 25, one arcing horn can in some devices be effectively employed in protecting the filamentary cathode. In other devices it may be desirable to employ more than two arcing horns.

Thus, it will be seen that I have provided an arrangement which can be relied upon for protecting the filamentary cathode against sputtering and burning that would otherwise result from fault currents exceeding the design fault current ratings of the tube. Additionally, my arrangement is relatively simple and inexpensive in construction, requires no design changes of the filamentary L cathode or of any of the other elements of the tube and does not affect the operating characteristics of the tube other than to improve its capacity for withstanding greater fault currents.

While I have shown and described a specific embodiment of my invention, I do not desire my invention to be limited to the particular forms shown and described, and I intend by the appended claims to cover all modifications Within the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A gaseous electric discharge device comprising in combination, an envelope containing an ionizable medium, an anode, a cathode structure cooperatively positioned with respect to said anode, said cathode structure comprising a thermionic cathode, a heat shield surrounding said thermionic cathode, said heat shield including an aperture for the passage therethrough of an arc stream extending between said anode and thermionic cathode, and a refractory metal cold cathode positioned in said shield and including a portion disposed in said are stream, said cold cathode bearing insulative material effective to collect ions from a plasma in said shield during fault current conduction, means'electrically connecting said cold cathode to said cathode structure whereby said cold cathode is adapted for the establishment thereon of cathode spots effective for attracting at least a portion of the arc in said device and thereby relieving said thermionic cathode of some of the duty of supporting the anode current during fault current conduction.

2. A gaseous electric discharge device according to claim ll, wherein said thermionic cathode comprises an alkaline earth metal oxide coating and said material on said cold cathode effective to accumulate ions is an oxide evaporated from said thermionic cathode.

3. A gaseous electric discharge device comprising an envelope containing an ionizable medium, an anode and a cathode structure cooperatively positioned with respect to said anode, said cathode structure comprising a heat shield surrounding said cathode and including an aperture for passage therethrough of an arc stream extending between said anode and thermionic cathode, said shield containing an ion plasma having a high ion density region located adjacent said thermionic cathode during fault current conduction across said device, and at least one refractory metal cold cathode including a wire-like portion disposed in said arc stream and in said high ion density region, said Wire-like portion being coated with an insulative material effective to collect ions from said plasma, whereby said cold cathode is adapted for the establishment of cathode spots thereon during fault current conduction for attracting at least a portion of the arc in said device and thereby relieving said filamentary thermionic cathode of some of the duty of supporting the anode current during fault current conduction.

4. A. gaseous electric discharge device comprising an envelope containing an ionizable medium, an anode and a cathode structure cooperatively positioned with respect to said anode, said cathode structure comprising a pair of support rods, an oxide coated filamentary thermionic cathode supported between said rods, a heat shield surrounding said thermionic cathode and including an aperture for passage therethrough of an arc stream extending between said anode and thermionic cathode, said shield containing an ion plasma having a high ion density region located adjacent said thermionic cathode during fault current conduction across said device, and at least one of said support rods having a refractory metal cold cathode mounted thereon and including a wire-like portion extending in said arc stream and in said high iondensity region, said wire-like portion being coated with an oxide evaporated from said thermionic cathode, Whereby said cold cathode is adapted for the establishment of cathode spots thereon during fault current conduction for attracting at least a portion of the arc in said device areas/1e and thereby relieving said filamentary thermionic cathode of some of the duty of supporting the anode current during fault current conduction.

5'. A gaseous electric discharge device comprising an envelope containing a mercury-vapor atmosphere, an anode and a cathode structure cooperatively positioned with respect to said anode, said cathode structure comprising a pair of support rods extending parallel to the axis of said device, a thermionic cathode comprising a refractory metal oxide coated mesh conductively connected between said support rods and extending transverse the axis of said device, a heat shield surrounding said thermionic cathode and including an aperture for said are stream and in said high ion density region parallel to said thermionic cathode, at least said opposed portions of said cold cathodes being coated with an oxide evaporated from said thermonic cathode, whereby said cold cathodes are adapted for the establishment of cathode spots thereon during fault current conduction for attract ing at least a portion of the arc in said device and thereby relieving said filamentary thermionic cathode of some of the duty of supporting the anode current during fault current conduction.

Reterences Cited by the Examiner UNITED STATES PATENTS 2,900,550 8/59 Fowler 3l3189 3,051,863 8/62 Knight 313-189 3,165,660 1/65 Menown 313-189 GEORGE N. WESTBY, Primary Examiner. 

1. A GASEOUS ELECTRIC DISCHARGE DEVICE COMPRISING IN COMBINATION, AN ENVELOPE CONTAINING AN IONIZABLE MEDIUM, AN ANODE, A CATHODE STRUCTURE COOPERATIVELY POSITIONED WITH RESPECT TO SAID ANODE, SAID CATHODE STRUCTURE COMPRISING A THERMIONIC CATHODE, A HEAT SHIELD SURROUNDING SAID THERMIONIC CATHODE, SAID HEAT SHIELD INCLUDING AN APERTURE FOR THE PASSAGE THERETHROUGH OF AN ARC STREAM EXTENDING BETWEEN SAID ANODE AND THERMIONIC CATHODE, AND A REFRACTORY METAL COLD CATHODE POSITIONED IN SAID SHIELD AND INCLUDING A PORTION DISPOSED IN SAID ARC STREAM, SAID COLD CATHODE BEARING INSULATIVE MATERIAL EFFECTIVE TO COLLECT IONS FROM A PLASMA IN SAID SHIELD DURING FAULT CURRENT CONDUCTION, MEANS ELECTRICALLY CONNECTING SAID COLD CATHODE TO SAID CATHODE STRUCTURE WHEREBY SAID COLD CATHODE IS ADAPTED FOR THE ESTABLISHMENT THEREON OF CATHODE SPOTS EFFECTIVE FOR ATTRACTING AT LEAST A PORTION OF THE ARC IN SAID DEVICE AND THEREBY RELIEVING SAID THERMIONIC CATHODE OF SOME OF THE DUTY OF SUPPORTING THE ANODE CURRENT DURING FAULT CURRENT CONDUCTION. 